The World Trade Center (WTC) attacks on September 11th, 2001 (9/11) provided a unique albeit tragic circumstance of occupational and environmental exposure to toxic agents. A prolonged followup is needed to fully understand the health effects in this population, as cases of many chronic diseases may occur after more than one decade from exposure. The use of biomarkers of early biologic effects, including DNA methylation, represents a powerful tool to identify conditions which may be associated with WTC exposure. However, for such markers to be useful, it is essential that they be validated against external measures of exposure. Detailed indicators of WTC exposure, called Exposure Ranking Indices (ERI) have been developed within the framework of a separate project on cancer risk in this population. Separate ERIs have been derived for acute and chronic WTC exposure. The overarching goal of the proposed research is to correlate ERIs to DNA methylation in a subset of 400 WTC responders attending the WTC Health Program Long Island Clinical Center of Excellence, in order to assess whether DNA methylation can be used as a biomarker of WTC exposure relevant to future risk of cancer and other chronic diseases. To reach this goal, global DNA methylation will be measured in blood samples of a subset of individuals for whom ERIs have already been estimated, and ERIs will be developed for a subset of WTC responders with available global DNA methylation results, for whom reliable ERI was not derived in the previous cancer project. A detailed analysis of the correlation between ERIs and DNA methylation will be then performed, including in particular the identification of candidate CpG sites associated with acute and chronic ERI, respectively. For the top CpG loci showing significant association with WTC exposure, array based detection of methylation levels will be validated with quantitative Sequenom EpiTYPER assays. The results of this analysis will have important implications in terms of developing a biomarker of WTC exposure which can be used in the future to determine the risk of cancer and other chronic diseases. In additio, the biomarker can be applied to other cohorts of WTC responders with lowquality environmental exposure data.